1. Field of the Invention
The invention relates to an image processing method that differentiates an image signal and further to a paint inspection method and paint inspection system that use the image processing method.
2. Description of the Related Art
An image signal acquired through an image pickup device may include an image portion that a user wants to keep and an image portion that is unnecessary for the user depending on the application of the image signal. The image portion that is unnecessary for the user has a large density change with respect to the surroundings. Thus, by detecting a portion having a large density change, it is possible to detect the image portion that is unnecessary for the user. Then, if the image portion that is unnecessary for the user may be detected, it is only necessary that the unnecessary image portion is removed.
Japanese Patent Application Publication No. 8-168004 (JP-A-8-168004) describes a method of forming a high-quality image close to an original picture by forming an appropriate edge for an image degraded because of noise, or the like. Japanese Patent Application Publication No. 2004-303075 (JP-A-2004-303075) describes an image processing technique for reducing low-frequency noise and high-frequency noise.
A known technique for calculating a density change includes Sobel filter, median filter, or the like. However, these techniques may possibly detect not only an image portion that the user wants to keep but also an unnecessary image portion.